1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, and more particularly to a system which performs a duty ratio control of an open/close valve disposed in an air intake side secondary air supply passage.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are well known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as an O.sub.2 sensor hereinafter) and the air-fuel ratio of the mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and for improvements of the fuel economy. As an example of the air-fuel ratio feedback control system, an air-intake side secondary air supply system for the feedback control is proposed, for example, in Japanese Patent Publication No. 55-3533. In this air intake side secondary air supply system, an open/close valve is disposed in an air intake side secondary air supply passage which communicates with the carburetor on the downstream side of the throttle valve; and the open/close valve is on-off controlled in response to the output signal level of the O.sub.2 sensor, so as to effect a "duty ratio control" of the supply of the air intake side secondary air. In conventional air intake side secondary air supply systems as described above, whether the air-fuel ratio of the mixture supplied to the engine is rich or lean with respect to a target air-fuel ratio is determined from an output signal level of the O.sub.2 sensor. The control of the open/close valve is performed in such a manner that a base valve open time within each duty cycle is decreased by a predetermined amount when a result of detection of air-fuel ratio is "lean". On the other hand, if the result of detection is "rich", the base valve open time period is increased by a predetermined amount. Then, a calculated value obtained by the integration operation described above is set as an output valve open period in which the open/close valve is opened, every cycle of the duty ratio control. In another case an amount of the correction value to be added to the base valve open period for the integration operation is decreased by a predetermined amount when the result of detection is "lean", and the amount of the correction value is increased by a predetermined amount when the result of detection is "rich". Then the correction value is added to the base valve open period, to produce the output valve open period.
With this type of conventional air intake side secondary air supply system, if the air-fuel ratio of the mixture supplied to the engine fluctuates wildly from the target air-fuel ratio due to a reason such as a rapid change in the engine operation, the duty ratio of the opening of the open/close valve was controlled over a wide range into a low end region (0-20% for example) or a high end region (90-100% for example).
However, in the case of usual open/close valves used for controlling the air flow, as shown in FIG. 1, the change in the amount of air flowing through the open/close valve will become very slight with respect to the change in the duty ratio in the low end part or in the high end part of the duty ratio. Therefore, to control the duty ratio of the supply of the air intake side secondary air over a wide range into the low end region or the high end part of the duty ratio, which was the case in the conventional design, is inefficient. Further, when the duty ratio of the supply of the air intake side secondary air is controlled into the high end region (90-100% for example), the air-fuel ratio of the mixture becomes over-lean, which in turn causes the deterioration of the driveability, and further the misfire of the ignition system.
Moreover, in the event that the O.sub.2 sensor fails, the output signal level of the O.sub.2 sensor will become saturated, as a result, the duty ratio of the opening and the closing of the open/close valve will stay around the value of 0% or 100%. In such case, the operation of the open/close valve during the feedback control operation of the air-fuel ratio will be stopped thereby setting the system out of control. As a result the air-fuel ratio is greatly differed from the target air-fuel ratio.